The present invention relates to devices which sense the magnitude of electrical current flowing through a conductor and cut off the flow of electricity when the current magnitude exceeds a given threshold, and more particularly to such devices which are capable of controlling the supply of electricity to a motor.
Large three-phase motors often are coupled to electricity supply lines by contactors which close and open to turn on and off the motor. The contactor, such as the one described in U.S. Pat. No. 3,536,868, in essence is a relay which is capable of switching large three-phase electric currents. In a simple installation, the contactor is operated by a signal applied to its control input by the operator opening or closing an electric switch. Thus, a large magnitude current to the motor can be controlled by switching a relatively small magnitude electric current to the control input.
As a safety feature, it is often desirable to include a mechanism which can detect when the current flowing through the contactor exceeds a safety threshold, and respond by disabling the contactor to cut off the flow of electricity to the motor. Such a device is commonly referred to as an overload relay. Previously this overload protection was provided by passing some or all of the motor current through a set of bi-metallic switch elements. The current flow heated bi-metallic material of the switch by an amount that corresponded to the magnitude of the current. When the motor current exceeded the safety threshold for a defined period of time, the heat caused the bi-metallic material to bend opening the switch contacts and terminating the application of current to the control input of the contactor. The level of current at which the bi-metallic switches open was difficult to calibrate and tended to become miscalibrated over time.
With the advent of solid state circuits, it became possible to incorporate an electronic sensing and control mechanism to perform the overload protection previously performed by the bi-metallic switching. The use of electronics provided a more reliable device and one that was easier to calibrate. However, such circuits often became relatively complex requiring low voltage power supplies and numerous components to adequately sense the electricity and activate the contactor mechanism.